1. Field of the invention
The present invention relates to a shank for a rotary and/or percussion working tool such as trepan, chisel, or boring bit for working stone, concrete, or brickwork.
2. Description of the Prior Art
Generally, a rotary and/or percussion working tool has a shank that extends along the working tool axis and is designed for a rotary and/or percussion hand-held power tool. For a random use of a multiple tool palette, the interface between a tool shank and a chuck of the power tool should be compatible at least within predetermined power classes. The presently used worldwide, in drill hammer systems, practically standardized shanks and associated chucks are disclosed, e.g., in German Publications DE 2551125A1 and DE 3716915A1. The disclosed shanks have a working too-side cylindrical guide surface, locking grooves which are axially closed at their ends adjacent to the free end surface of the shank, and trapezoidal entraining grooves which are open toward the free end surface of the shank. The associated chuck has at least one radially displaceable locking body that engages in a locking groove, limiting an axial displacement of the shank in the chuck.
The practically standardized shanks and chucks, which are disclosed in the German Publication DE 2551125A1, have a guide diameter of 10 mm, two identical, diametrically opposite, locking grooves, and two identical, diametrically opposite entraining grooves which are symmetrically distributed over the shank circumference. The entraining grooves, which are slightly longer than the locking grooves, are adjoined at their working tool side by a guide surface that extends up to the working tool-side end of the chuck and that does not contribute to the transmission of a torque. These shanks were initially designed for a drill diameter up to 17 mm and are, thus, associated with a range of small, not very powerful, hammer drills with a power smaller than 600 W. The more powerful hand-held power tools, in particular, hammer drills transmit, in predetermined operational modes, high torques to the working tool. Meanwhile, a practical operational field of the hammer drills expanded to a drill diameter of 30 mm. A particularly high torque is transmitted to the tool during its withdrawal from a work piece, in particular of a tool that became jammed in the bore. It has been shown that an increase of the drill diameter above 17 mm leads to damages, specifically, to breaking of the shank in the region of a locking groove and to destructions in the interior of the chuck. These damages are aggravated by the fact that the broken ends remain in the interior of the hammer drill and can be removed from the chuck only by dismounting the front portion of the hammer drill. Even in the case when during the use of drills having a larger diameter, a break does not take place, a plastic deformation of the shank takes place which leads to a non-uniform noticeable wear of the chuck. Because of this, working tools are often very difficult to withdraw from a chuck.
German Publications DE 3716195A1 discloses a practically standardized shank and chuck with a guide diameter 18 mm, with the shank having two, identical, diametrically opposite, locking grooves and arranged there between, three symmetrically distributed, entraining grooves one of which is arranged in one axial half of the shank and two of which are arranged in another shank half. This shank is designed for transmitting large torques in powerful, large hammer drills. Whit this shank, the problems, which were discussed in the preceding paragraph with reference to high power classes or torques are, naturally, likewise occur.
Drills with a guide diameter of 18 mm but with a smaller drill diameter of 14 mm have a poor blow transmission. Furthermore, such unproportional tools are not economically in manufacture.
The loads that act on such shanks includes the following components. On one hand, the shank is loaded by the blow energy of the hammer drill, On the other hand, a torsional load produced by the chuck rotary wedges, which is determined by a torque acting on the bit, is transmitted to the entraining grooves. The torque is particularly high when, e.g., the bit is wedged during drilling in reinforced concrete. An additional load is applied in case of wedging in a reinforcing metal, e.g., during an attempt to pull the drill back. In addition, a load, which is applied by the locking body at the axial locking end of the locking groove, acts on the rear cross-section of the locking groove. A multi-year experience has shown that these multiple loads endanger, in particular, the cross-section of the shank in the region of the axial locking end of the shank. The mechanical fracturing causes of this are suspected to lie in local, pronounced, multi-axis stress state which produces a local stiffening that is caused by a transverse contraction. The stiffening represents a primary fracture initiator, limiting the fatigue strength of the shank which is subjected to alternating stresses.
U.S. Pat. No. 4,655,651 disclosed a shank having a working tool-side cylindrical guide surface, three axial pointed grooves opening toward the free end and surface of the shank, and a plurality of axial locking grooves offset relative to the pointed grooves and having each a locking shoulder remote from the shank free end surface, with the locking grooves being at least, partially super-imposed on the pointed grooves. The three pointed grooves, each of which has an angle of 120° between the legs, serve for receiving the clamping jaws of a three-jaw chuck. Such tapering, inclined to a radius by 60°, legs are not suitable for transmissions of high torques that occur, in particular, in tool blocks. In addition, the locking grooves, reduce the leg surface available for transmission of a torque.
Accordingly, an object of the present invention is a shank capable of both transmitting high torques without being damaged, and providing an optimal blow pulse transmission.